1. Field of the Invention
The invention relates to an arrangement for influencing the radial clearance of the blading in axial-flow, highly loaded compressors of gas turbines.
2. Discussion of Background
Due to the present high economic and ecological requirements, higher and higher efficiencies are aimed at in modern thermal turbomachines, for example industrial gas turbines, which also leads, inter alia, to higher pressure and temperature ratios in the compressor.
This higher loading of the compressor can be realized with shorter blades, but this leads to an increase in the relative radial clearance, related to the blade height, of the moving blades and thus to a reduction in the pumping limit of the compressor.
In addition, it is known that the gap losses in the gap between the moving blades of the rotor of thermal turbomachines and the stator parts opposite them have a great effect on the efficiency of the machine. The greater the gap losses, the lower the efficiency. Attempts are therefore made to keep the gap as narrow as possible during operation, but large enough for the cooling phase.
The radial clearance is caused on the one hand by inaccuracies during production and assembly but on the other hand in particular by the different thermal behavior of the rotor and the blade carrier.
The rotor is usually more solid than the blade carrier on account of the strength requirements. Since both the heat-transfer conditions and usually the material for both parts are similar, the rotor is therefore thermally substantially slower. There is also the fact that not only are the thermal expansions of rotor and blade carrier different in the operating state, but also their variation with time is different, especially during start-up and stopping of the machine. In interaction with the centrifugal force, this results in a minimum radial clearance during the start in the hot state just after the stopping and a maximum radial clearance during the start in the cold state.
It is known from CH 639 171 that a reduction in the radial gap in axial turbomachines can be achieved through the use of cover rings which are in one piece in the peripheral direction, are arranged one behind the other in the axial direction of the machine and are centered by centering wedges. These rings have temperatures which are rotationally symmetric all round at each operating point, for which reason the circular form is retained and the cover rings thus remain centered to the rotor axis in the event of deformations as a result of temperature changes, so that the minimum gap size, to be taken into account in the design, between rotor wheel and cover ring can be reduced. If the cover rings are used at the same time for mounting and holding the guide-blade rims arranged between them, the gap, likewise causing power losses, between the inner guide-blade end (possibly shroud band) and the rotor can be minimized, too. Since the cover rings should be made of a material having as low a coefficient of thermal expansion as possible, for example an iron-nickel alloy, they are relatively expensive. A further disadvantage with this prior art is the complicated construction which results from the fastening of the centering wedges and their centering.
DE 33 05 170 C2 discloses a turbomachine casing having an outer casing wall and an inner casing wall subdivided into sectors in the peripheral direction, both being connected to one another via detachable fastenings, in which the sectors of the inner casing wall are formed by retaining rails which have a clearance space at their ends in the peripheral direction and which are provided with radial supporting extensions and form intermediate spaces between the inner and outer casing wall which are filled with thermal insulating material. A disadvantage with this prior art is the splitting-up of the blade carrier into an outer and an inner casing wall, in which case only thin guide-blade roots can be accommodated on account of the thin inner casing wall, and the forces which occur are transmitted via the root and the inner casing wall to the outer casing wall.